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..yeah, thought so ;)
So you have the exact same problem as me...I usually have 400-500MB free, yet the game closes usually after about 5-10min being active.
After restarting my device and clearing all caches, I sometimes even get 20 minutes.
Nothing you can do here I'm afraid, the game needs too much memory and get's shut down by the operating system or some memory exception thrown by the game....
The only solution I know of is getting a device having more RAM (>2GB should be fine)
That, or the more complicated way of expanding your RAM via swapfile. But this requires:
1. Rooting your device
2. Checking if your device-kernel is supported (free app)
3. If compatible, installing this app: ROEHSOFT RAM Expander (8.99€)
Since I wanted to try the latter anyways, I can write you if it worked or not...